1. Technical Field
The present invention relates to a modulation circuit and, more particularly, the present invention relates to a modulation circuit that generates an output signal obtained by frequency-modulating an input signal with a local signal.
2. Related Art
Conventionally, in fields relating to measuring devices or wireless communication, a modulator is known that shifts a frequency of an input signal (a base band signal) according to a local signal. Furthermore, as such a modulator, a circuit is known that uses a ring FET or a resistive FET.
A modulator using the resistive FET inputs a local signal into a gate terminal of a FET having a grounded source and inputs an input signal into a drain terminal of the FET having a grounded source. Through such a configuration, a signal obtained by multiplying the local signal by the input signal is output from the drain terminal of the FET. In other words, the resistive FET has a relatively favorable IP3 (third intercept point) characteristic because the source terminal is grounded and has a small circuit size because only a single FET is used in the configuration.
In a modulator using the resistive FET, however, the input signal is supplied to the same terminal of the FET (the drain) as the output signal is taken from. Therefore, in the modulator described above, the input signal and the output signal are separated by a filter. For example, in a case where an output signal obtained by shifting a frequency of the input signal to a higher frequency is generated (up-conversion), the input signal and the output signal can be separated by disposing a low-pass filter between the input terminal and the drain terminal and disposing a high-pass filter between the output terminal and the drain terminal. Furthermore, in a case where an output signal obtained by shifting a frequency of the input signal to a lower frequency is generated (down-conversion), the input signal and the output signal can be separated by disposing the high-pass filter between the input terminal and the drain terminal and disposing the low-pass filter between the output terminal and the drain terminal.
In the manner described above, the modulator using the resistive FET has a different filter configuration in a case of up-conversion than in a case of down-conversion. Therefore, a single modulator cannot have both a down-converting function and an up-converting function.
On the other hand, a modulator using a ring FET is inferior to the modulator using the resistive FET in regards to the IP3 characteristics and the circuit size. In the modulator using the ring FET, however, it is not necessary to include a filter for separating the input signal and the output signal because the input signal and the output signal are transmitted with different lines, so that a single modulator can have both the down-converting function and the up-converting function. However, a modulator having both the down-converting function and the up-converting function, favorable IP3 characteristics, and a small circuit size is not yet known.